Data Communications and Networking 5e - PDF

CHAPTER 1 Introduction D ata communications and networking have changed the way we do business and the way we live. Business decisions have to be made ever more quickly, and the deci- sion makers require immediate access to accur...

CHAPTER 1 Introduction D ata communications and networking have changed the way we do business and the way we live. Business decisions have to be made ever more quickly, and the deci- sion makers require immediate access to accurate information. Why wait a week for that report from Europe to arrive by mail when it could appear almost instantaneously through computer networks? Businesses today rely on computer networks and internet- works. Data communication and networking have found their way not only through busi- ness and personal communication, they have found many applications in political and social issues. People have found how to communicate with other people in the world to express their social and political opinions and problems. Communities in the world are not isolated anymore. But before we ask how quickly we can get hooked up, we need to know how net- works operate, what types of technologies are available, and which design best fills which set of needs. This chapter paves the way for the rest of the book. It is divided into five sections. ❑ The first section introduces data communications and defines their components and the types of data exchanged. It also shows how different types of data are rep- resented and how data is flowed through the network. ❑ The second section introduces networks and defines their criteria and structures. It introduces four different network topologies that are encountered throughout the book. ❑ The third section discusses different types of networks: LANs, WANs, and inter- networks (internets). It also introduces the Internet, the largest internet in the world. The concept of switching is also introduced in this section to show how small networks can be combined to create larger ones. ❑ The fourth section covers a brief history of the Internet. The section is divided into three eras: early history, the birth of the Internet, and the issues related to the Inter- net today. This section can be skipped if the reader is familiar with this history. ❑ The fifth section covers standards and standards organizations. The section covers Internet standards and Internet administration. We refer to these standards and organizations throughout the book. 3 4 PART I OVERVIEW 1.1 DATA COMMUNICATIONS When we communicate, we are sharing information. This sharing can be local or remote. Between individuals, local communication usually occurs face to face, while remote communication takes place over distance. The term telecommunication, which includes telephony, telegraphy, and television, means communication at a distance (tele is Greek for “far”). The word data refers to information presented in whatever form is agreed upon by the parties creating and using the data. Data communications are the exchange of data between two devices via some form of transmission medium such as a wire cable. For data communications to occur, the communicating devices must be part of a communication system made up of a com- bination of hardware (physical equipment) and software (programs). The effectiveness of a data communications system depends on four fundamental characteristics: deliv- ery, accuracy, timeliness, and jitter. 1. Delivery. The system must deliver data to the correct destination. Data must be received by the intended device or user and only by that device or user. 2. Accuracy. The system must deliver the data accurately. Data that have been altered in transmission and left uncorrected are unusable. 3. Timeliness. The system must deliver data in a timely manner. Data delivered late are useless. In the case of video and audio, timely delivery means delivering data as they are produced, in the same order that they are produced, and without signifi- cant delay. This kind of delivery is called real-time transmission. 4. Jitter. Jitter refers to the variation in the packet arrival time. It is the uneven delay in the delivery of audio or video packets. For example, let us assume that video packets are sent every 30 ms. If some of the packets arrive with 30-ms delay and others with 40-ms delay, an uneven quality in the video is the result. 1.1.1 Components A data communications system has five components (see Figure 1.1). Figure 1.1 Five components of data communication Rule 1: Rule 1: Rule 2: Rule 2: Protocol Protocol...... Rule n: Rule n: Message Sender Receiver Transmission medium 1. Message. The message is the information (data) to be communicated. Popular forms of information include text, numbers, pictures, audio, and video. 2. Sender. The sender is the device that sends the data message. It can be a com- puter, workstation, telephone handset, video camera, and so on.  CHAPTER 1 INTRODUCTION 5 3. Receiver. The receiver is the device that receives the message. It can be a com- puter, workstation, telephone handset, television, and so on. 4. Transmission medium. The transmission medium is the physical path by which a message travels from sender to receiver. Some examples of transmission media include twisted-pair wire, coaxial cable, fiber-optic cable, and radio waves. 5. Protocol. A protocol is a set of rules that govern data communications. It repre- sents an agreement between the communicating devices. Without a protocol, two devices may be connected but not communicating, just as a person speaking French cannot be understood by a person who speaks only Japanese. 1.1.2 Data Representation Information today comes in different forms such as text, numbers, images, audio, and video. Text In data communications, text is represented as a bit pattern, a sequence of bits (0s or 1s). Different sets of bit patterns have been designed to represent text symbols. Each set is called a code, and the process of representing symbols is called coding. Today, the prevalent coding system is called Unicode, which uses 32 bits to represent a symbol or character used in any language in the world. The American Standard Code for Infor- mation Interchange (ASCII), developed some decades ago in the United States, now constitutes the first 127 characters in Unicode and is also referred to as Basic Latin. Appendix A includes part of the Unicode. Numbers Numbers are also represented by bit patterns. However, a code such as ASCII is not used to represent numbers; the number is directly converted to a binary number to simplify mathematical operations. Appendix B discusses several different numbering systems. Images Images are also represented by bit patterns. In its simplest form, an image is composed of a matrix of pixels (picture elements), where each pixel is a small dot. The size of the pixel depends on the resolution. For example, an image can be divided into 1000 pixels or 10,000 pixels. In the second case, there is a better representation of the image (better resolution), but more memory is needed to store the image. After an image is divided into pixels, each pixel is assigned a bit pattern. The size and the value of the pattern depend on the image. For an image made of only black- and-white dots (e.g., a chessboard), a 1-bit pattern is enough to represent a pixel. If an image is not made of pure white and pure black pixels, we can increase the size of the bit pattern to include gray scale. For example, to show four levels of gray scale, we can use 2-bit patterns. A black pixel can be represented by 00, a dark gray pixel by 01, a light gray pixel by 10, and a white pixel by 11. There are several methods to represent color images. One method is called RGB, so called because each color is made of a combination of three primary colors: red, green, and blue. The intensity of each color is measured, and a bit pattern is assigned to 6 PART I OVERVIEW it. Another method is called YCM, in which a color is made of a combination of three other primary colors: yellow, cyan, and magenta. Audio Audio refers to the recording or broadcasting of sound or music. Audio is by nature different from text, numbers, or images. It is continuous, not discrete. Even when we use a microphone to change voice or music to an electric signal, we create a continuous signal. We will learn more about audio in Chapter 26. Video Video refers to the recording or broadcasting of a picture or movie. Video can either be produced as a continuous entity (e.g., by a TV camera), or it can be a combination of images, each a discrete entity, arranged to convey the idea of motion. We will learn more about video in Chapter 26. 1.1.3 Data Flow Communication between two devices can be simplex, half-duplex, or full-duplex as shown in Figure 1.2. Figure 1.2 Data flow (simplex, half-duplex, and full-duplex) Direction of data Mainframe a. Simplex Monitor Direction of data at time 1 Direction of data at time 2 b. Half-duplex Direction of data all the time c. Full-duplex Simplex In simplex mode, the communication is unidirectional, as on a one-way street. Only one of the two devices on a link can transmit; the other can only receive (see Figure 1.2a). Keyboards and traditional monitors are examples of simplex devices. The key- board can only introduce input; the monitor can only accept output. The simplex mode can use the entire capacity of the channel to send data in one direction.  CHAPTER 1 INTRODUCTION 7 Half-Duplex In half-duplex mode, each station can both transmit and receive, but not at the same time. When one device is sending, the other can only receive, and vice versa (see Figure 1.2b). The half-duplex mode is like a one-lane road with traffic allowed in both direc- tions. When cars are traveling in one direction, cars going the other way must wait. In a half-duplex transmission, the entire capacity of a channel is taken over by whichever of the two devices is transmitting at the time. Walkie-talkies and CB (citizens band) radios are both half-duplex systems. The half-duplex mode is used in cases where there is no need for communication in both directions at the same time; the entire capacity of the channel can be utilized for each direction. Full-Duplex In full-duplex mode (also called duplex), both stations can transmit and receive simul- taneously (see Figure 1.2c). The full-duplex mode is like a two-way street with traffic flowing in both direc- tions at the same time. In full-duplex mode, signals going in one direction share the capacity of the link with signals going in the other direction. This sharing can occur in two ways: Either the link must contain two physically separate transmission paths, one for sending and the other for receiving; or the capacity of the channel is divided between signals traveling in both directions. One common example of full-duplex communication is the telephone network. When two people are communicating by a telephone line, both can talk and listen at the same time. The full-duplex mode is used when communication in both directions is required all the time. The capacity of the channel, however, must be divided between the two directions. 1.2 NETWORKS A network is the interconnection of a set of devices capable of communication. In this definition, a device can be a host (or an end system as it is sometimes called) such as a large computer, desktop, laptop, workstation, cellular phone, or security system. A device in this definition can also be a connecting device such as a router, which con- nects the network to other networks, a switch, which connects devices together, a modem (modulator-demodulator), which changes the form of data, and so on. These devices in a network are connected using wired or wireless transmission media such as cable or air. When we connect two computers at home using a plug-and-play router, we have created a network, although very small. 1.2.1 Network Criteria A network must be able to meet a certain number of criteria. The most important of these are performance, reliability, and security. 8 PART I OVERVIEW Performance Performance can be measured in many ways, including transit time and response time. Transit time is the amount of time required for a message to travel from one device to another. Response time is the elapsed time between an inquiry and a response. The per- formance of a network depends on a number of factors, including the number of users, the type of transmission medium, the capabilities of the connected hardware, and the efficiency of the software. Performance is often evaluated by two networking metrics: throughput and delay. We often need more throughput and less delay. However, these two criteria are often contradictory. If we try to send more data to the network, we may increase throughput but we increase the delay because of traffic congestion in the network. Reliability In addition to accuracy of delivery, network reliability is measured by the frequency of failure, the time it takes a link to recover from a failure, and the network’s robustness in a catastrophe. Security Network security issues include protecting data from unauthorized access, protecting data from damage and development, and implementing policies and procedures for recovery from breaches and data losses. 1.2.2 Physical Structures Before discussing networks, we need to define some network attributes. Type of Connection A network is two or more devices connected through links. A link is a communications pathway that transfers data from one device to another. For visualization purposes, it is simplest to imagine any link as a line drawn between two points. For communication to occur, two devices must be connected in some way to the same link at the same time. There are two possible types of connections: point-to-point and multipoint. Point-to-Point A point-to-point connection provides a dedicated link between two devices. The entire capacity of the link is reserved for transmission between those two devices. Most point-to-point connections use an actual length of wire or cable to connect the two ends, but other options, such as microwave or satellite links, are also possible (see Figure 1.3a). When we change television channels by infrared remote control, we are establishing a point-to-point connection between the remote control and the television’s control system. Multipoint A multipoint (also called multidrop) connection is one in which more than two spe- cific devices share a single link (see Figure 1.3b).  CHAPTER 1 INTRODUCTION 9 Figure 1.3 Types of connections: point-to-point and multipoint Link a. Point-to-point Link Mainframe b. Multipoint In a multipoint environment, the capacity of the channel is shared, either spatially or temporally. If several devices can use the link simultaneously, it is a spatially shared connection. If users must take turns, it is a timeshared connection. Physical Topology The term physical topology refers to the way in which a network is laid out physically. Two or more devices connect to a link; two or more links form a topology. The topology of a network is the geometric representation of the relationship of all the links and linking devices (usually called nodes) to one another. There are four basic topologies possible: mesh, star, bus, and ring. Mesh Topology In a mesh topology, every device has a dedicated point-to-point link to every other device. The term dedicated means that the link carries traffic only between the two devices it connects. To find the number of physical links in a fully connected mesh net- work with n nodes, we first consider that each node must be connected to every other node. Node 1 must be connected to n – 1 nodes, node 2 must be connected to n – 1 nodes, and finally node n must be connected to n – 1 nodes. We need n (n – 1) physical links. However, if each physical link allows communication in both directions (duplex mode), we can divide the number of links by 2. In other words, we can say that in a mesh topology, we need n (n – 1) / 2 duplex-mode links. To accommodate that many links, every device on the network must have n – 1 input/output (I/O) ports (see Figure 1.4) to be connected to the other n – 1 stations. A mesh offers several advantages over other network topologies. First, the use of dedicated links guarantees that each connection can carry its own data load, thus elimi- nating the traffic problems that can occur when links must be shared by multiple devices. Second, a mesh topology is robust. If one link becomes unusable, it does not incapacitate the entire system. Third, there is the advantage of privacy or security. When every message travels along a dedicated line, only the intended recipient sees it. Physical boundaries prevent other users from gaining access to messages. Finally, point-to-point links make fault identification and fault isolation easy. Traffic can be routed to avoid links with suspected problems. This facility enables the network manager to discover the precise location of the fault and aids in finding its cause and solution. 10 PART I OVERVIEW Figure 1.4 A fully connected mesh topology (five devices) n=5 10 links. The main disadvantages of a mesh are related to the amount of cabling and the number of I/O ports required. First, because every device must be connected to every other device, installation and reconnection are difficult. Second, the sheer bulk of the wiring can be greater than the available space (in walls, ceilings, or floors) can accom- modate. Finally, the hardware required to connect each link (I/O ports and cable) can be prohibitively expensive. For these reasons a mesh topology is usually implemented in a limited fashion, for example, as a backbone connecting the main computers of a hybrid network that can include several other topologies. One practical example of a mesh topology is the connection of telephone regional offices in which each regional office needs to be connected to every other regional office. Star Topology In a star topology, each device has a dedicated point-to-point link only to a central con- troller, usually called a hub. The devices are not directly linked to one another. Unlike a mesh topology, a star topology does not allow direct traffic between devices. The con- troller acts as an exchange: If one device wants to send data to another, it sends the data to the controller, which then relays the data to the other connected device (see Figure 1.5). Figure 1.5 A star topology connecting four stations Hub A star topology is less expensive than a mesh topology. In a star, each device needs only one link and one I/O port to connect it to any number of others. This factor also makes it easy to install and reconfigure. Far less cabling needs to be housed, and  CHAPTER 1 INTRODUCTION 11 additions, moves, and deletions involve only one connection: between that device and the hub. Other advantages include robustness. If one link fails, only that link is affected. All other links remain active. This factor also lends itself to easy fault identification and fault isolation. As long as the hub is working, it can be used to monitor link problems and bypass defective links. One big disadvantage of a star topology is the dependency of the whole topology on one single point, the hub. If the hub goes down, the whole system is dead. Although a star requires far less cable than a mesh, each node must be linked to a central hub. For this reason, often more cabling is required in a star than in some other topologies (such as ring or bus). The star topology is used in local-area networks (LANs), as we will see in Chapter 13. High-speed LANs often use a star topology with a central hub. Bus Topology The preceding examples all describe point-to-point connections. A bus topology, on the other hand, is multipoint. One long cable acts as a backbone to link all the devices in a network (see Figure 1.6). Figure 1.6 A bus topology connecting three stations Drop line Drop line Drop line Cable end Cable end Tap Tap Tap Nodes are connected to the bus cable by drop lines and taps. A drop line is a con- nection running between the device and the main cable. A tap is a connector that either splices into the main cable or punctures the sheathing of a cable to create a contact with the metallic core. As a signal travels along the backbone, some of its energy is trans- formed into heat. Therefore, it becomes weaker and weaker as it travels farther and far- ther. For this reason there is a limit on the number of taps a bus can support and on the distance between those taps. Advantages of a bus topology include ease of installation. Backbone cable can be laid along the most efficient path, then connected to the nodes by drop lines of various lengths. In this way, a bus uses less cabling than mesh or star topologies. In a star, for example, four network devices in the same room require four lengths of cable reaching all the way to the hub. In a bus, this redundancy is eliminated. Only the backbone cable stretches through the entire facility. Each drop line has to reach only as far as the near- est point on the backbone. Disadvantages include difficult reconnection and fault isolation. A bus is usually designed to be optimally efficient at installation. It can therefore be difficult to add new devices. Signal reflection at the taps can cause degradation in quality. This degradation can be controlled by limiting the number and spacing of devices connected to a given 12 PART I OVERVIEW length of cable. Adding new devices may therefore require modification or replacement of the backbone. In addition, a fault or break in the bus cable stops all transmission, even between devices on the same side of the problem. The damaged area reflects signals back in the direction of origin, creating noise in both directions. Bus topology was the one of the first topologies used in the design of early local- area networks. Traditional Ethernet LANs can use a bus topology, but they are less pop- ular now for reasons we will discuss in Chapter 13. Ring Topology In a ring topology, each device has a dedicated point-to-point connection with only the two devices on either side of it. A signal is passed along the ring in one direction, from device to device, until it reaches its destination. Each device in the ring incorporates a repeater. When a device receives a signal intended for another device, its repeater regenerates the bits and passes them along (see Figure 1.7). Figure 1.7 A ring topology connecting six stations Repeater Repeater Repeater Repeater Repeater Repeater A ring is relatively easy to install and reconfigure. Each device is linked to only its immediate neighbors (either physically or logically). To add or delete a device requires changing only two connections. The only constraints are media and traffic consider- ations (maximum ring length and number of devices). In addition, fault isolation is sim- plified. Generally, in a ring a signal is circulating at all times. If one device does not receive a signal within a specified period, it can issue an alarm. The alarm alerts the network operator to the problem and its location. However, unidirectional traffic can be a disadvantage. In a simple ring, a break in the ring (such as a disabled station) can disable the entire network. This weakness can be solved by using a dual ring or a switch capable of closing off the break. Ring topology was prevalent when IBM introduced its local-area network, Token Ring. Today, the need for higher-speed LANs has made this topology less popular.  CHAPTER 1 INTRODUCTION 13 1.3 NETWORK TYPES After defining networks in the previous section and discussing their physical structures, we need to discuss different types of networks we encounter in the world today. The crite- ria of distinguishing one type of network from another is difficult and sometimes confus- ing. We use a few criteria such as size, geographical coverage, and ownership to make this distinction. After discussing two types of networks, LANs and WANs, we define switch- ing, which is used to connect networks to form an internetwork (a network of networks). 1.3.1 Local Area Network A local area network (LAN) is usually privately owned and connects some hosts in a single office, building, or campus. Depending on the needs of an organization, a LAN can be as simple as two PCs and a printer in someone’s home office, or it can extend throughout a company and include audio and video devices. Each host in a LAN has an identifier, an address, that uniquely defines the host in the LAN. A packet sent by a host to another host carries both the source host’s and the destination host’s addresses. In the past, all hosts in a network were connected through a common cable, which meant that a packet sent from one host to another was received by all hosts. The intended recipient kept the packet; the others dropped the packet. Today, most LANs use a smart connecting switch, which is able to recognize the destination address of the packet and guide the packet to its destination without sending it to all other hosts. The switch allevi- ates the traffic in the LAN and allows more than one pair to communicate with each other at the same time if there is no common source and destination among them. Note that the above definition of a LAN does not define the minimum or maximum number of hosts in a LAN. Figure 1.8 shows a LAN using either a common cable or a switch. Figure 1.8 An isolated LAN in the past and today Host 1 Host 2 Host 3 Host 4 Host 5 Host 6 Host 7 Host 8 a. LAN with a common cable (past) Legend Host 1 Host 2 Host 3 Host 4 A host (of any type) A switch A cable tap A cable end Switch The common cable A connection Host 5 Host 6 Host 7 Host 8 b. LAN with a switch (today) 14 PART I OVERVIEW LANs are discussed in more detail in Part III of the book. When LANs were used in isolation (which is rare today), they were designed to allow resources to be shared between the hosts. As we will see shortly, LANs today are connected to each other and to WANs (discussed next) to create communication at a wider level. 1.3.2 Wide Area Network A wide area network (WAN) is also an interconnection of devices capable of communica- tion. However, there are some differences between a LAN and a WAN. A LAN is normally limited in size, spanning an office, a building, or a campus; a WAN has a wider geographi- cal span, spanning a town, a state, a country, or even the world. A LAN interconnects hosts; a WAN interconnects connecting devices such as switches, routers, or modems. A LAN is normally privately owned by the organization that uses it; a WAN is normally created and run by communication companies and leased by an organization that uses it. We see two distinct examples of WANs today: point-to-point WANs and switched WANs. Point-to-Point WAN A point-to-point WAN is a network that connects two communicating devices through a trans- mission media (cable or air). We will see examples of these WANs when we discuss how to connect the networks to one another. Figure 1.9 shows an example of a point-to-point WAN. Figure 1.9 A point-to-point WAN A connecting device Legend Connecting medium To another To another network network Switched WAN A switched WAN is a network with more than two ends. A switched WAN, as we will see shortly, is used in the backbone of global communication today. We can say that a switched WAN is a combination of several point-to-point WANs that are connected by switches. Figure 1.10 shows an example of a switched WAN. Figure 1.10 A switched WAN To another To another network network To another To another Legend network network A switch Connecting medium To another To another network network To another To another network network  CHAPTER 1 INTRODUCTION 15 WANs are discussed in more detail in Part II of the book. Internetwork Today, it is very rare to see a LAN or a WAN in isolation; they are connected to one another. When two or more networks are connected, they make an internetwork, or internet. As an example, assume that an organization has two offices, one on the east coast and the other on the west coast. Each office has a LAN that allows all employees in the office to communicate with each other. To make the communication between employ- ees at different offices possible, the management leases a point-to-point dedicated WAN from a service provider, such as a telephone company, and connects the two LANs. Now the company has an internetwork, or a private internet (with lowercase i). Communication between offices is now possible. Figure 1.11 shows this internet. Figure 1.11 An internetwork made of two LANs and one point-to-point WAN Point-to-point R1 WAN R2 LAN Router Router LAN West coast ofce East coast ofce When a host in the west coast office sends a message to another host in the same office, the router blocks the message, but the switch directs the message to the destination. On the other hand, when a host on the west coast sends a message to a host on the east coast, router R1 routes the packet to router R2, and the packet reaches the destination. Figure 1.12 (see next page) shows another internet with several LANs and WANs connected. One of the WANs is a switched WAN with four switches. 1.3.3 Switching An internet is a switched network in which a switch connects at least two links together. A switch needs to forward data from a network to another network when required. The two most common types of switched networks are circuit-switched and packet-switched networks. We discuss both next. Circuit-Switched Network In a circuit-switched network, a dedicated connection, called a circuit, is always available between the two end systems; the switch can only make it active or inactive. Figure 1.13 shows a very simple switched network that connects four telephones to each end. We have used telephone sets instead of computers as an end system because circuit switching was very common in telephone networks in the past, although part of the telephone network today is a packet-switched network. In Figure 1.13, the four telephones at each side are connected to a switch. The switch connects a telephone set at one side to a telephone set at the other side. The thick 16 PART I OVERVIEW Figure 1.12 A heterogeneous network made of four WANs and three LANs Point-to-point Modem WAN Modem Resident Switched WAN Router Point-to-point WAN Router Router Point-to-point WAN LAN Router LAN Figure 1.13 A circuit-switched network Low-capacity line High-capacity line Switch Switch line connecting two switches is a high-capacity communication line that can handle four voice communications at the same time; the capacity can be shared between all pairs of telephone sets. The switches used in this example have forwarding tasks but no storing capability. Let us look at two cases. In the first case, all telephone sets are busy; four people at one site are talking with four people at the other site; the capacity of the thick line is fully used. In the second case, only one telephone set at one side is connected to a tele- phone set at the other side; only one-fourth of the capacity of the thick line is used. This means that a circuit-switched network is efficient only when it is working at its full capacity; most of the time, it is inefficient because it is working at partial capacity. The reason that we need to make the capacity of the thick line four times the capacity of each voice line is that we do not want communication to fail when all telephone sets at one side want to be connected with all telephone sets at the other side.  CHAPTER 1 INTRODUCTION 17 Packet-Switched Network In a computer network, the communication between the two ends is done in blocks of data called packets. In other words, instead of the continuous communication we see between two telephone sets when they are being used, we see the exchange of individ- ual data packets between the two computers. This allows us to make the switches func- tion for both storing and forwarding because a packet is an independent entity that can be stored and sent later. Figure 1.14 shows a small packet-switched network that con- nects four computers at one site to four computers at the other site. Figure 1.14 A packet-switched network Low-capacity line High-capacity line Queue Queue Router Router A router in a packet-switched network has a queue that can store and forward the packet. Now assume that the capacity of the thick line is only twice the capacity of the data line connecting the computers to the routers. If only two computers (one at each site) need to communicate with each other, there is no waiting for the packets. However, if packets arrive at one router when the thick line is already working at its full capacity, the packets should be stored and forwarded in the order they arrived. The two simple examples show that a packet-switched network is more efficient than a circuit- switched network, but the packets may encounter some delays. In this book, we mostly discuss packet-switched networks. In Chapter 18, we discuss packet-switched networks in more detail and discuss the performance of these networks. 1.3.4 The Internet As we discussed before, an internet (note the lowercase i) is two or more networks that can communicate with each other. The most notable internet is called the Internet (uppercase I ), and is composed of thousands of interconnected networks. Figure 1.15 shows a conceptual (not geographical) view of the Internet. The figure shows the Internet as several backbones, provider networks, and cus- tomer networks. At the top level, the backbones are large networks owned by some communication companies such as Sprint, Verizon (MCI), AT&T, and NTT. The back- bone networks are connected through some complex switching systems, called peering points. At the second level, there are smaller networks, called provider networks, that use the services of the backbones for a fee. The provider networks are connected to backbones and sometimes to other provider networks. The customer networks are 18 PART I OVERVIEW Figure 1.15 The Internet today Customer Customer Customer Customer network network network network Provider Provider network network Peering point Peering point Backbones Provider Provider Provider network network network Customer Customer Customer Customer Customer Customer network network network network network network networks at the edge of the Internet that actually use the services provided by the Inter- net. They pay fees to provider networks for receiving services. Backbones and provider networks are also called Internet Service Providers (ISPs). The backbones are often referred to as international ISPs; the provider net- works are often referred to as national or regional ISPs. 1.3.5 Accessing the Internet The Internet today is an internetwork that allows any user to become part of it. The user, however, needs to be physically connected to an ISP. The physical connection is normally done through a point-to-point WAN. In this section, we briefly describe how this can happen, but we postpone the technical details of the connection until Chapters 14 and 16. Using Telephone Networks Today most residences and small businesses have telephone service, which means they are connected to a telephone network. Since most telephone networks have already connected themselves to the Internet, one option for residences and small businesses to connect to the Internet is to change the voice line between the residence or business and the telephone center to a point-to-point WAN. This can be done in two ways. ❑ Dial-up service. The first solution is to add to the telephone line a modem that converts data to voice. The software installed on the computer dials the ISP and imitates making a telephone connection. Unfortunately, the dial-up service is  CHAPTER 1 INTRODUCTION 19 very slow, and when the line is used for Internet connection, it cannot be used for telephone (voice) connection. It is only useful for small residences. We discuss dial-up service in Chapter 14. ❑ DSL Service. Since the advent of the Internet, some telephone companies have upgraded their telephone lines to provide higher speed Internet services to resi- dences or small businesses. The DSL service also allows the line to be used simul- taneously for voice and data communication. We discuss DSL in Chapter 14. Using Cable Networks More and more residents over the last two decades have begun using cable TV services instead of antennas to receive TV broadcasting. The cable companies have been upgrading their cable networks and connecting to the Internet. A residence or a small business can be connected to the Internet by using this service. It provides a higher speed connection, but the speed varies depending on the number of neighbors that use the same cable. We discuss the cable networks in Chapter 14. Using Wireless Networks Wireless connectivity has recently become increasingly popular. A household or a small business can use a combination of wireless and wired connections to access the Internet. With the growing wireless WAN access, a household or a small business can be connected to the Internet through a wireless WAN. We discuss wireless access in Chapter 16. Direct Connection to the Internet A large organization or a large corporation can itself become a local ISP and be con- nected to the Internet. This can be done if the organization or the corporation leases a high-speed WAN from a carrier provider and connects itself to a regional ISP. For example, a large university with several campuses can create an internetwork and then connect the internetwork to the Internet. 1.4 INTERNET HISTORY Now that we have given an overview of the Internet, let us give a brief history of the Internet. This brief history makes it clear how the Internet has evolved from a private network to a global one in less than 40 years. 1.4.1 Early History There were some communication networks, such as telegraph and telephone networks, before 1960. These networks were suitable for constant-rate communication at that time, which means that after a connection was made between two users, the encoded message (telegraphy) or voice (telephony) could be exchanged. A computer network, on the other hand, should be able to handle bursty data, which means data received at variable rates at different times. The world needed to wait for the packet-switched network to be invented. 20 PART I OVERVIEW Birth of Packet-Switched Networks The theory of packet switching for bursty traffic was first presented by Leonard Kleinrock in 1961 at MIT. At the same time, two other researchers, Paul Baran at Rand Institute and Donald Davies at National Physical Laboratory in England, published some papers about packet-switched networks. ARPANET In the mid-1960s, mainframe computers in research organizations were stand-alone devices. Computers from different manufacturers were unable to communicate with one another. The Advanced Research Projects Agency (ARPA) in the Department of Defense (DOD) was interested in finding a way to connect computers so that the researchers they funded could share their findings, thereby reducing costs and eliminat- ing duplication of effort. In 1967, at an Association for Computing Machinery (ACM) meeting, ARPA pre- sented its ideas for the Advanced Research Projects Agency Network (ARPANET), a small network of connected computers. The idea was that each host computer (not necessarily from the same manufacturer) would be attached to a specialized computer, called an interface message processor (IMP). The IMPs, in turn, would be connected to each other. Each IMP had to be able to communicate with other IMPs as well as with its own attached host. By 1969, ARPANET was a reality. Four nodes, at the University of California at Los Angeles (UCLA), the University of California at Santa Barbara (UCSB), Stanford Research Institute (SRI), and the University of Utah, were connected via the IMPs to form a network. Software called the Network Control Protocol (NCP) provided com- munication between the hosts. 1.4.2 Birth of the Internet In 1972, Vint Cerf and Bob Kahn, both of whom were part of the core ARPANET group, collaborated on what they called the Internetting Project. They wanted to link dissimilar networks so that a host on one network could communicate with a host on another. There were many problems to overcome: diverse packet sizes, diverse inter- faces, and diverse transmission rates, as well as differing reliability requirements. Cerf and Kahn devised the idea of a device called a gateway to serve as the intermediary hardware to transfer data from one network to another. TCP/IP Cerf and Kahn’s landmark 1973 paper outlined the protocols to achieve end-to-end delivery of data. This was a new version of NCP. This paper on transmission control protocol (TCP) included concepts such as encapsulation, the datagram, and the func- tions of a gateway. A radical idea was the transfer of responsibility for error correction from the IMP to the host machine. This ARPA Internet now became the focus of the communication effort. Around this time, responsibility for the ARPANET was handed over to the Defense Communication Agency (DCA). In October 1977, an internet consisting of three different networks (ARPANET, packet radio, and packet satellite) was successfully demonstrated. Communication between networks was now possible.  CHAPTER 1 INTRODUCTION 21 Shortly thereafter, authorities made a decision to split TCP into two protocols: Trans- mission Control Protocol (TCP) and Internet Protocol (IP). IP would handle datagram routing while TCP would be responsible for higher level functions such as segmentation, reassembly, and error detection. The new combination became known as TCP/IP. In 1981, under a Defence Department contract, UC Berkeley modified the UNIX operating system to include TCP/IP. This inclusion of network software along with a popular operating system did much for the popularity of internetworking. The open (non-manufacturer-specific) implementation of the Berkeley UNIX gave every manu- facturer a working code base on which they could build their products. In 1983, authorities abolished the original ARPANET protocols, and TCP/IP became the official protocol for the ARPANET. Those who wanted to use the Internet to access a computer on a different network had to be running TCP/IP. MILNET In 1983, ARPANET split into two networks: Military Network (MILNET) for military users and ARPANET for nonmilitary users. CSNET Another milestone in Internet history was the creation of CSNET in 1981. Computer Science Network (CSNET) was a network sponsored by the National Science Founda- tion (NSF). The network was conceived by universities that were ineligible to join ARPANET due to an absence of ties to the Department of Defense. CSNET was a less expensive network; there were no redundant links and the transmission rate was slower. By the mid-1980s, most U.S. universities with computer science departments were part of CSNET. Other institutions and companies were also forming their own net- works and using TCP/IP to interconnect. The term Internet, originally associated with government-funded connected networks, now referred to the connected networks using TCP/IP protocols. NSFNET With the success of CSNET, the NSF in 1986 sponsored the National Science Founda- tion Network (NSFNET), a backbone that connected five supercomputer centers located throughout the United States. Community networks were allowed access to this backbone, a T-1 line (see Chapter 6) with a 1.544-Mbps data rate, thus providing connec- tivity throughout the United States. In 1990, ARPANET was officially retired and replaced by NSFNET. In 1995, NSFNET reverted back to its original concept of a research network. ANSNET In 1991, the U.S. government decided that NSFNET was not capable of supporting the rapidly increasing Internet traffic. Three companies, IBM, Merit, and Verizon, filled the void by forming a nonprofit organization called Advanced Network & Services (ANS) to build a new, high-speed Internet backbone called Advanced Network Services Network (ANSNET). 22 PART I OVERVIEW 1.4.3 Internet Today Today, we witness a rapid growth both in the infrastructure and new applications. The Internet today is a set of pier networks that provide services to the whole world. What has made the Internet so popular is the invention of new applications. World Wide Web The 1990s saw the explosion of Internet applications due to the emergence of the World Wide Web (WWW). The Web was invented at CERN by Tim Berners-Lee. This inven- tion has added the commercial applications to the Internet. Multimedia Recent developments in the multimedia applications such as voice over IP (telephony), video over IP (Skype), view sharing (YouTube), and television over IP (PPLive) has increased the number of users and the amount of time each user spends on the network. We discuss multimedia in Chapter 28. Peer-to-Peer Applications Peer-to-peer networking is also a new area of communication with a lot of potential. We introduce some peer-to-peer applications in Chapter 29. 1.5 STANDARDS AND ADMINISTRATION In the discussion of the Internet and its protocol, we often see a reference to a standard or an administration entity. In this section, we introduce these standards and adminis- tration entities for those readers that are not familiar with them; the section can be skipped if the reader is familiar with them. 1.5.1 Internet Standards An Internet standard is a thoroughly tested specification that is useful to and adhered to by those who work with the Internet. It is a formalized regulation that must be followed. There is a strict procedure by which a specification attains Internet standard status. A spec- ification begins as an Internet draft. An Internet draft is a working document (a work in progress) with no official status and a six-month lifetime. Upon recommendation from the Internet authorities, a draft may be published as a Request for Comment (RFC). Each RFC is edited, assigned a number, and made available to all interested parties. RFCs go through maturity levels and are categorized according to their requirement level. Maturity Levels An RFC, during its lifetime, falls into one of six maturity levels: proposed standard, draft standard, Internet standard, historic, experimental, and informational (see Figure 1.16). ❑ Proposed Standard. A proposed standard is a specification that is stable, well understood, and of sufficient interest to the Internet community. At this level, the specification is usually tested and implemented by several different groups.  CHAPTER 1 INTRODUCTION 23 Figure 1.16 Maturity levels of an RFC Internet draft Experimental Proposed standard Informational Six months and two tries Draft standard Four months and two tries Internet standard Historic ❑ Draft Standard. A proposed standard is elevated to draft standard status after at least two successful independent and interoperable implementations. Barring diffi- culties, a draft standard, with modifications if specific problems are encountered, normally becomes an Internet standard. ❑ Internet Standard. A draft standard reaches Internet standard status after demon- strations of successful implementation. ❑ Historic. The historic RFCs are significant from a historical perspective. They either have been superseded by later specifications or have never passed the neces- sary maturity levels to become an Internet standard. ❑ Experimental. An RFC classified as experimental describes work related to an experimental situation that does not affect the operation of the Internet. Such an RFC should not be implemented in any functional Internet service. ❑ Informational. An RFC classified as informational contains general, historical, or tutorial information related to the Internet. It is usually written by someone in a non-Internet organization, such as a vendor. Requirement Levels RFCs are classified into five requirement levels: required, recommended, elective, lim- ited use, and not recommended. ❑ Required. An RFC is labeled required if it must be implemented by all Internet systems to achieve minimum conformance. For example, IP and ICMP (Chapter 19) are required protocols. ❑ Recommended. An RFC labeled recommended is not required for minimum conformance; it is recommended because of its usefulness. For example, FTP (Chapter 26) and TELNET (Chapter 26) are recommended protocols. ❑ Elective. An RFC labeled elective is not required and not recommended. However, a system can use it for its own benefit. 24 PART I OVERVIEW ❑ Limited Use. An RFC labeled limited use should be used only in limited situations. Most of the experimental RFCs fall under this category. ❑ Not Recommended. An RFC labeled not recommended is inappropriate for gen- eral use. Normally a historic (deprecated) RFC may fall under this category. RFCs can be found at http://www.rfc-editor.org. 1.5.2 Internet Administration The Internet, with its roots primarily in the research domain, has evolved and gained a broader user base with significant commercial activity. Various groups that coordinate Internet issues have guided this growth and development. Appendix G gives the addresses, e-mail addresses, and telephone numbers for some of these groups. Figure 1.17 shows the general organization of Internet administration. Figure 1.17 Internet administration ISOC IAB IRTF IETF IRSG IESG Area Area RG RG RG RG WG WG WG WG ISOC The Internet Society (ISOC) is an international, nonprofit organization formed in 1992 to provide support for the Internet standards process. ISOC accomplishes this through maintaining and supporting other Internet administrative bodies such as IAB, IETF, IRTF, and IANA (see the following sections). ISOC also promotes research and other scholarly activities relating to the Internet. IAB The Internet Architecture Board (IAB) is the technical advisor to the ISOC. The main purposes of the IAB are to oversee the continuing development of the TCP/IP Protocol Suite and to serve in a technical advisory capacity to research members of the Internet community. IAB accomplishes this through its two primary components, the Internet Engineering Task Force (IETF) and the Internet Research Task Force (IRTF). Another responsibility of the IAB is the editorial management of the RFCs, described  CHAPTER 1 INTRODUCTION 25 earlier. IAB is also the external liaison between the Internet and other standards organi- zations and forums. IETF The Internet Engineering Task Force (IETF) is a forum of working groups managed by the Internet Engineering Steering Group (IESG). IETF is responsible for identifying operational problems and proposing solutions to these problems. IETF also develops and reviews specifications intended as Internet standards. The working groups are col- lected into areas, and each area concentrates on a specific topic. Currently nine areas have been defined. The areas include applications, protocols, routing, network manage- ment next generation (IPng), and security. IRTF The Internet Research Task Force (IRTF) is a forum of working groups managed by the Internet Research Steering Group (IRSG). IRTF focuses on long-term research top- ics related to Internet protocols, applications, architecture, and technology. 1.6 END-CHAPTER MATERIALS 1.6.1 Recommended Reading For more details about subjects discussed in this chapter, we recommend the following books. The items enclosed in brackets [...] refer to the reference list at the end of the book. Books The introductory materials covered in this chapter can be found in [Sta04] and [PD03]. [Tan03] also discusses standardization. 1.6.2 Key Terms Advanced Network Services Network full-duplex mode (ANSNET) half-duplex mode Advanced Research Projects Agency (ARPA) hub Advanced Research Projects Agency Network image (ARPANET) internet American Standard Code for Information Internet Interchange (ASCII) Internet Architecture Board (IAB) audio Internet draft backbone Internet Engineering Task Force (IETF) Basic Latin Internet Research Task Force (IRTF) bus topology Internet Service Provider (ISP) circuit-switched network Internet Society (ISOC) code Internet standard Computer Science Network (CSNET) internetwork data local area network (LAN) data communications mesh topology delay message 26 PART I OVERVIEW Military Network (MILNET) ring topology multipoint or multidrop connection simplex mode National Science Foundation Network star topology (NSFNET) switched network network TCP/IP protocol suite node telecommunication packet throughput packet-switched network Transmission Control Protocol/ Internet performance Protocol (TCP/IP) physical topology transmission medium point-to-point connection Unicode protocol video Request for Comment (RFC) wide area network (WAN) RGB YCM 1.6.3 Summary Data communications are the transfer of data from one device to another via some form of transmission medium. A data communications system must transmit data to the correct destination in an accurate and timely manner. The five components that make up a data communications system are the message, sender, receiver, medium, and protocol. Text, numbers, images, audio, and video are different forms of information. Data flow between two devices can occur in one of three ways: simplex, half-duplex, or full-duplex. A network is a set of communication devices connected by media links. In a point- to-point connection, two and only two devices are connected by a dedicated link. In a multipoint connection, three or more devices share a link. Topology refers to the physical or logical arrangement of a network. Devices may be arranged in a mesh, star, bus, or ring topology. A network can be categorized as a local area network or a wide area network. A LAN is a data communication system within a building, plant, or campus, or between nearby buildings. A WAN is a data communication system spanning states, countries, or the whole world. An internet is a network of networks. The Internet is a collection of many separate networks. The Internet history started with the theory of packet switching for bursty traffic. The history continued when The ARPA was interested in finding a way to connect computers so that the researchers they funded could share their findings, resulting in the creation of ARPANET. The Internet was born when Cerf and Kahn devised the idea of a device called a gateway to serve as the intermediary hardware to transfer data from one network to another. The TCP/IP protocol suite paved the way for creation of today’s Internet. The invention of WWW, the use of multimedia, and peer-to-peer com- munication helps the growth of the Internet. An Internet standard is a thoroughly tested specification. An Internet draft is a working document with no official status and a six-month lifetime. A draft may be published as a Request for Comment (RFC). RFCs go through maturity levels and are categorized according to their requirement level. The Internet administration has  CHAPTER 1 INTRODUCTION 27 evolved with the Internet. ISOC promotes research and activities. IAB is the technical advisor to the ISOC. IETF is a forum of working groups responsible for operational problems. IRTF is a forum of working groups focusing on long-term research topics. 1.7 PRACTICE SET 1.7.1 Quizzes A set of interactive quizzes for this chapter can be found on the book website. It is strongly recommended that the student take the quizzes to check his/her understanding of the materials before continuing with the practice set. 1.7.2 Questions Q1-1. Identify the five components of a data communications system. Q1-2. What are the three criteria necessary for an effective and efficient network? Q1-3. What are the advantages of a multipoint connection over a point-to-point one? Q1-4. What are the two types of line configuration? Q1-5. Categorize the four basic topologies in terms of line configuration. Q1-6. What is the difference between half-duplex and full-duplex transmission modes? Q1-7. Name the four basic network topologies, and cite an advantage of each type. Q1-8. For n devices in a network, what is the number of cable links required for a mesh, ring, bus, and star topology? Q1-9. What are some of the factors that determine whether a communication system is a LAN or WAN? Q1-10. What is an internet? What is the Internet? Q1-11. Why are protocols needed? Q1-12. In a LAN with a link-layer switch (Figure 1.8b), Host 1 wants to send a mes- sage to Host 3. Since communication is through the link-layer switch, does the switch need to have an address? Explain. Q1-13. How many point-to-point WANs are needed to connect n LANs if each LAN should be able to directly communicate with any other LAN? Q1-14. When we use local telephones to talk to a friend, are we using a circuit- switched network or a packet-switched network? Q1-15. When a resident uses a dial-up or DLS service to connect to the Internet, what is the role of the telephone company? Q1-16. What is the first principle we discussed in this chapter for protocol layering that needs to be followed to make the communication bidirectional? Q1-17. Explain the difference between an Internet draft and a proposed standard. Q1-18. Explain the difference between a required RFC and a recommended RFC. Q1-19. Explain the difference between the duties of the IETF and IRTF. 28 PART I OVERVIEW 1.7.3 Problems P1-1. What is the maximum number of characters or symbols that can be repre- sented by Unicode? P1-2. A color image uses 16 bits to represent a pixel. What is the maximum number of different colors that can be represented? P1-3. Assume six devices are arranged in a mesh topology. How many cables are needed? How many ports are needed for each device? P1-4. For each of the following four networks, discuss the consequences if a con- nection fails. a. Five devices arranged in a mesh topology b. Five devices arranged in a star topology (not counting the hub) c. Five devices arranged in a bus topology d. Five devices arranged in a ring topology P1-5. We have two computers connected by an Ethernet hub at home. Is this a LAN or a WAN? Explain the reason. P1-6. In the ring topology in Figure 1.7, what happens if one of the stations is unplugged? P1-7. In the bus topology in Figure 1.6, what happens if one of the stations is unplugged? P1-8. Performance is inversely related to delay. When we use the Internet, which of the following applications are more sensitive to delay? a. Sending an e-mail b. Copying a file c. Surfing the Internet P1-9. When a party makes a local telephone call to another party, is this a point-to- point or multipoint connection? Explain the answer. P1-10. Compare the telephone network and the Internet. What are the similarities? What are the differences? 1.8 SIMULATION EXPERIMENTS 1.8.1 Applets One of the ways to show the network protocols in action or visually see the solution to some examples is through the use of interactive animation. We have created some Java applets to show some of the main concepts discussed in this chapter. It is strongly rec- ommended that the students activate these applets on the book website and carefully examine the protocols in action. However, note that applets have been created only for some chapters, not all (see the book website). 1.8.2 Lab Assignments Experiments with networks and network equipment can be done using at least two methods. In the first method, we can create an isolated networking laboratory and use  CHAPTER 1 INTRODUCTION 29 networking hardware and software to simulate the topics discussed in each chapter. We can create an internet and send and receive packets from any host to another. The flow of packets can be observed and the performance can be measured. Although the first method is more effective and more instructional, it is expensive to implement and not all institutions are ready to invest in such an exclusive laboratory. In the second method, we can use the Internet, the largest network in the world, as our virtual laboratory. We can send and receive packets using the Internet. The exis- tence of some free-downloadable software allows us to capture and examine the pack- ets exchanged. We can analyze the packets to see how theoretical aspects of networking are put into action. Although the second method may not be as effective as the first method, in that we cannot control and change the packet routes to see how the Internet behaves, the method is much cheaper to implement. It does not need a physical net- working lab; it can be implemented using our desktop or laptop. The required software is also free to download. There are many programs and utilities available for Windows and UNIX operating systems that allow us to sniff, capture, trace, and analyze packets that are exchanged between our computer and the Internet. Some of these, such as Wireshark and Ping- Plotter, have graphical user interface (GUI); others, such as traceroute, nslookup, dig, ipconfig, and ifconfig, are network administration command-line utilities. Any of these programs and utilities can be a valuable debugging tool for network administrators and educational tool for computer network students. In this book, we mostly use Wireshark for lab assignments, although we occasion- ally use other tools. It captures live packet data from a network interface and displays them with detailed protocol information. Wireshark, however, is a passive analyzer. It only “measures” things from the network without manipulating them; it doesn’t send packets on the network or perform other active operations. Wireshark is not an intru- sion detection tool either. It does not give warning about any network intrusion. It, nevertheless, can help network administrators or network security engineers to figure out what is going on inside a network and to troubleshoot network problems. In addi- tion to being an indispensable tool for network administrators and security engineers, Wireshark is a valuable tool for protocol developers, who may use it to debug protocol implementations, and a great educational tool for computer networking students who can use it to see details of protocol operations in real time. However, note that we can use lab assignments only with a few chapters. Lab1-1. In this lab assignment we learn how to download and install Wireshark. The instructions for downloading and installing the software are posted on the book website in the lab section for Chapter 1. In this document, we also dis- cuss the general idea behind the software, the format of its window, and how to use it. The full study of this lab prepares the student to use Wireshark in the lab assignments for other chapters.  CHAPTER 2 Network Models T he second chapter is a preparation for the rest of the book. The next five parts of the book is devoted to one of the layers in the TCP/IP protocol suite. In this chapter, we first discuss the idea of network models in general and the TCP/IP protocol suite in particular. Two models have been devised to define computer network operations: the TCP/IP protocol suite and the OSI model. In this chapter, we first discuss a general subject, protocol layering, which is used in both models. We then concentrate on the TCP/IP protocol suite, on which the book is based. The OSI model is briefly discuss for comparison with the TCP/IP protocol suite. ❑ The first section introduces the concept of protocol layering using two scenarios. The section also discusses the two principles upon which the protocol layering is based. The first principle dictates that each layer needs to have two opposite tasks. The second principle dictates that the corresponding layers should be identical. The section ends with a brief discussion of logical connection between two identi- cal layers in protocol layering. Throughout the book, we need to distinguish between logical and physical connections. ❑ The second section discusses the five layers of the TCP/IP protocol suite. We show how packets in each of the five layers (physical, data-link, network, transport, and application) are named. We also mention the addressing mechanism used in each layer. Each layer of the TCP/IP protocol suite is a subject of a part of the book. In other words, each layer is discussed in several chapters; this section is just an intro- duction and preparation. ❑ The third section gives a brief discussion of the OSI model. This model was never implemented in practice, but a brief discussion of the model and its comparison with the TCP/IP protocol suite may be useful to better understand the TCP/IP pro- tocol suite. In this section we also give a brief reason for the OSI model’s lack of success. 31 32 PART I OVERVIEW 2.1 PROTOCOL LAYERING We defined the term protocol in Chapter 1. In data communication and networking, a protocol defines the rules that both the sender and receiver and all intermediate devices need to follow to be able to communicate effectively. When communication is simple, we may need only one simple protocol; when the communication is complex, we may need to divide the task between different layers, in which case we need a protocol at each layer, or protocol layering. 2.1.1 Scenarios Let us develop two simple scenarios to better understand the need for protocol layering. First Scenario In the first scenario, communication is so simple that it can occur in only one layer. Assume Maria and Ann are neighbors with a lot of common ideas. Communication between Maria and Ann takes place in one layer, face to face, in the same language, as shown in Figure 2.1. Figure 2.1 A single-layer protocol Maria Ann Layer 1 Listen/Talk Listen/Talk Layer 1 Air Even in this simple scenario, we can see that a set of rules needs to be followed. First, Maria and Ann know that they should greet each other when they meet. Second, they know that they should confine their vocabulary to the level of their friendship. Third, each party knows that she should refrain from speaking when the other party is speaking. Fourth, each party knows that the conversation should be a dialog, not a monolog: both should have the opportunity to talk about the issue. Fifth, they should exchange some nice words when they leave. We can see that the protocol used by Maria and Ann is different from the commu- nication between a professor and the students in a lecture hall. The communication in the second case is mostly monolog; the professor talks most of the time unless a student has a question, a situation in which the protocol dictates that she should raise her hand and wait for permission to speak. In this case, the communication is normally very for- mal and limited to the subject being taught. Second Scenario In the second scenario, we assume that Ann is offered a higher-level position in her company, but needs to move to another branch located in a city very far from Maria. The two friends still want to continue their communication and exchange ideas because  CHAPTER 2 NETWORK MODELS 33 they have come up with an innovative project to start a new business when they both retire. They decide to continue their conversation using regular mail through the post office. However, they do not want their ideas to be revealed by other people if the let- ters are intercepted. They agree on an encryption/decryption technique. The sender of the letter encrypts it to make it unreadable by an intruder; the receiver of the letter decrypts it to get the original letter. We discuss the encryption/decryption methods in Chapter 31, but for the moment we assume that Maria and Ann use one technique that makes it hard to decrypt the letter if one does not have the key for doing so. Now we can say that the communication between Maria and Ann takes place in three layers, as shown in Figure 2.2. We assume that Ann and Maria each have three machines (or robots) that can perform the task at each layer. Figure 2.2 A three-layer protocol Maria Ann Layer 3 Listen/Talk Listen/Talk Layer 3 Identical objects Plaintext Plaintext Layer 2 Encrypt/Decrypt Encrypt/Decrypt Layer 2 Identical objects Ciphertext Ciphertext Send mail/ Send mail/ Layer 1 Layer 1 receive mail receive mail Identical objects Mail Mail US Post US Post Postal carrier facility Let us assume that Maria sends the first letter to Ann. Maria talks to the machine at the third layer as though the machine is Ann and is listening to her. The third layer machine listens to what Maria says and creates the plaintext (a letter in English), which is passed to the second layer machine. The second layer machine takes the plaintext, encrypts it, and creates the ciphertext, which is passed to the first layer machine. The first layer machine, presumably a robot, takes the ciphertext, puts it in an envelope, adds the sender and receiver addresses, and mails it. At Ann’s side, the first layer machine picks up the letter from Ann’s mail box, rec- ognizing the letter from Maria by the sender address. The machine takes out the cipher- text from the envelope and delivers it to the second layer machine. The second layer machine decrypts the message, creates the plaintext, and passes the plaintext to the third-layer machine. The third layer machine takes the plaintext and reads it as though Maria is speaking. 34 PART I OVERVIEW Protocol layering enables us to divide a complex task into several smaller and sim- pler tasks. For example, in Figure 2.2, we could have used only one machine to do the job of all three machines. However, if Maria and Ann decide that the encryption/ decryption done by the machine is not enough to protect their secrecy, they would have to change the whole machine. In the present situation, they need to change only the sec- ond layer machine; the other two can remain the same. This is referred to as modularity. Modularity in this case means independent layers. A layer (module) can be defined as a black box with inputs and outputs, without concern about how inputs are changed to outputs. If two machines provide the same outputs when given the same inputs, they can replace each other. For example, Ann and Maria can buy the second layer machine from two different manufacturers. As long as the two machines create the same cipher- text from the same plaintext and vice versa, they do the job. One of the advantages of protocol layering is that it allows us to separate the services from the implementation. A layer needs to be able to receive a set of ser- vices from the lower layer and to give the services to the upper layer; we don’t care about how the layer is implemented. For example, Maria may decide not to buy the machine (robot) for the first layer; she can do the job herself. As long as Maria can do the tasks provided by the first layer, in both directions, the communication system works. Another advantage of protocol layering, which cannot be seen in our simple exam- ples but reveals itself when we discuss protocol layering in the Internet, is that commu- nication does not always use only two end systems; there are intermediate systems that need only some layers, but not all layers. If we did not use protocol layering, we would have to make each intermediate system as complex as the end systems, which makes the whole system more expensive. Is there any disadvantage to protocol layering? One can argue that having a single layer makes the job easier. There is no need for each layer to provide a service to the upper layer and give service to the lower layer. For example, Ann and Maria could find or build one machine that could do all three tasks. However, as mentioned above, if one day they found that their code was broken, each would have to replace the whole machine with a new one instead of just changing the machine in the second layer. 2.1.2 Principles of Protocol Layering Let us discuss two principles of protocol layering. First Principle The first principle dictates that if we want bidirectional communication, we need to make each layer so that it is able to perform two opposite tasks, one in each direction. For example, the third layer task is to listen (in one direction) and talk (in the other direction). The second layer needs to be able to encrypt and decrypt. The first layer needs to send and receive mail. Second Principle The second principle that we need to follow in protocol layering is that the two objects under each layer at both sites should be identical. For example, the object under layer 3 at both sites should be a plaintext letter. The object under layer 2 at  CHAPTER 2 NETWORK MODELS 35 both sites should be a ciphertext letter. The object under layer 1 at both sites should be a piece of mail. 2.1.3 Logical Connections After following the above two principles, we can think about logical connection between each layer as shown in Figure 2.3. This means that we have layer-to-layer communication. Maria and Ann can think that there is a logical (imaginary) connection at each layer through which they can send the object created from that layer. We will see that the concept of logical connection will help us better understand the task of lay- ering we encounter in data communication and networking. Figure 2.3 Logical connection between peer layers Maria Ann Layer 3 Talk/Listen Listen/Talk Layer 3 Logical connection Plaintext Plaintext Layer 2 Encrypt/Decrypt Encrypt/Decrypt Layer 2 Logical connection Ciphertext Ciphertext Send mail/ Send mail/ Layer 1 Layer 1 receive mail receive mail Mail Logical connection Mail 2.2 TCP/IP PROTOCOL SUITE Now that we know about the concept of protocol layering and the logical communica- tion between layers in our second scenario, we can introduce the TCP/IP (Transmission Control Protocol/Internet Protocol). TCP/IP is a protocol suite (a set of protocols orga- nized in different layers) used in the Internet today. It is a hierarchical protocol made up of interactive modules, each of which provides a specific functionality. The term hier- archical means that each upper level protocol is supported by the services provided by one or more lower level protocols. The original TCP/IP protocol suite was defined as four software layers built upon the hardware. Today, however, TCP/IP is thought of as a five-layer model. Figure 2.4 shows both configurations. 2.2.1 Layered Architecture To show how the layers in the TCP/IP protocol suite are involved in communication between two hosts, we assume that we want to use the suite in a small internet made up of three LANs (links), each with a link-layer switch. We also assume that the links are connected by one router, as shown in Figure 2.5. 36 PART I OVERVIEW Figure 2.4 Layers in the TCP/IP protocol suite Application Application Layer 5 Transport Transport Layer 4 Internet Network Layer 3 Network Interface Data link Layer 2 Hardware Devices Physical Layer 1 a. Original layers b. Layers used in this book Figure 2.5 Communication through an internet Source (A) Destination (B) Application Application Transport Router Transport Network Switch Network Switch Network Data link Data link Data link Data link Data link Data link Physical Physical Physical Physical Physical Physical Communication from A to B Router A Link 1 Link 2 B Link 3 C Let us assume that computer A communicates with computer B. As the figure shows, we have five communicating devices in this communication: source host (computer A), the link-layer switch in link 1, the router, the link-layer switch in link 2, and the destination host (computer B). Each device is involved with a set of layers depending on the role of the device in the internet. The two hosts are involved in all five layers; the source host needs to create a message in the application layer and send it down the layers so that it is physically sent to the destination host. The destination host needs to receive the communication at the physical layer and then deliver it through the other layers to the application layer.  CHAPTER 2 NETWORK MODELS 37 The router is involved in only three layers; there is no transport or application layer in a router as long as the router is used only for routing. Although a router is always involved in one network layer, it is involved in n combinations of link and physical lay- ers in which n is the number of links the router is connected to. The reason is that each link may use its own data-link or physical protocol. For example, in the above figure, the router is involved in three links, but the message sent from source A to destination B is involved in two links. Each link may be using different link-layer and physical-layer protocols; the router needs to receive a packet from link 1 based on one pair of proto- cols and deliver it to link 2 based on another pair of protocols. A link-layer switch in a link, however, is involved only in two layers, data-link and physical. Although each switch in the above figure has two different connections, the connections are in the same link, which uses only one set of protocols. This means that, unlike a router, a link-layer switch is involved only in one data-link and one physical layer. 2.2.2 Layers in the TCP/IP Protocol Suite After the above introduction, we briefly discuss the functions and duties of layers in the TCP/IP protocol suite. Each layer is discussed in detail in the next five parts of the book. To better understand the duties of each layer, we need to think about the logical connections between layers. Figure 2.6 shows logical connections in our sim- ple internet. Figure 2.6 Logical connections between layers of the TCP/IP protocol suite Source Destination host host Logical connections Application Application Transport Transport Network Network Data link Data link Physical Physical Switch Router Switch LAN LAN Router Source Link 1 Link 2 Destination host To link 3 host Using logical connections makes it easier for us to think about the duty of each layer. As the figure shows, the duty of the application, transport, and network layers is end-to-end. However, the duty of the data-link and physical layers is hop-to-hop, in which a hop is a host or router. In other words, the domain of duty of the top three layers is th